How do I find all content that use a specific custom component?

All builder content entries save a dictionary in its meta of all the custom components used and a count of them:

meta: {
  componentsUsed: {
    ProductBox: 1,
    ProductCollectionGrid: 2

So you can use Mongo-style queries to check for all content that has a specific component:{{your api key}}&query.meta.componentsUsed.{{ component name }}.$exists=true